A relay system environment can be broken down into a simple relay environment in which a relay simply transfers user data to a destination, and a user relay environment in which a relay transmits both user data and the relay system's data. Hereinafter, a transmission technique for mutually cooperative relaying between users in a user relay environment will be referred to as a mutually cooperative relaying (MCR) technique.
The conventional MCR techniques can be implemented in a variety of forms, and the MCR techniques that have been studied so far can be classified into three types.
The first type is an orthogonal signaling-based MCR technique (V. Mahinthan and J. W. Mark, ‘A Simple Cooperative Diversity based on Orthogonal Signaling’, IEEE WCNC, Vol. 2, pp. 1012-1017, March 2005). According to the method, one of two users uses the I-channel (in-phase) component of a quadrature phase shift keying (QPSK) plane in order to transmit the user's data, and the other user transmits data to be relayed by using the Q-channel (quadrature-phase) component.
However, when mutually cooperative relaying is performed by using the orthogonal signaling-based MCR technique, the transmission rate is reduced to half that of direct transmission. For example, if a user 1 can transmit 2-bit data when mutually co-operative relaying is not used (that is, in the case of direct transmission), when mutually cooperative relaying is used, the user 1 should relay data of a user 2 for mutually cooperative relaying, and therefore the user 1 can transmit only 1-bit data of his/her own.
The second type is a superposition-based MCR technique (E. G. Larsson and B. R. Vojcic, ‘Cooperative Transmit Diversity based on Superposition Modulation’, IEEE Communication Letters, Vol. 9, No. 9, pp. 778-780, September 2005). According to the method, each of two users combines symbols of his/her data and data to be relayed, transmits the symbols, and then, in a destination, a desired symbol is detected by using the difference between a symbol received in a current time unit and a symbol detected in the previous time unit. However, this method is greatly affected by transmission power, and in a worst case, 2 signaling points among 4 signaling points may disappear.
Finally, the third type is a space-time block code (STBC)-based MCR technique (P. Tarasak, Hlaing Minn, and V. K. Bhargava, ‘Differential Modulation for Two-User co-operative Diversity systems’, IEEE Comm., Vol. 23, Issue 9, pp. 1891-1900, September 2005). According to the method, the STBC is applied to the MCR technique and in order to obtain a diversity with a second order, an Alamouti's code is used. However, in order for each user to transmit one symbol, three time slots should be used in the method.